Why beta carbon hydrogen is easily replaced​

Iknow #2 is d. i just need to know why

Which of these expressions are correct variations of the combined gas law? p1v1t2 = p2v2t1 both

Which best describes the present-day universe? opaque, expanding very slowly, stars produce heavy elements transparent, expanding at an accelerated rate, stars produce heavy elements opaque, expanding at an accelerated rate, stars produce only hydrogen and helium transparent, expanding very slowly, stars produce only hydrogen and helium